Avoid repeatedly opening /dev/null

[nojb]

Recently I started getting the following errors when building moderately-sized workspaces:

Error: open: /dev/null: Too many open files
Error: open: /tmp/dune528db4.output: Too many open files
Error: open: /tmp/dune93b269.output: Too many open files
Error: open: /tmp/duneb4d3c8.output: Too many open files
Error: open: /tmp/dune472f36.output: Too many open files
Error: open: /tmp/dune62787e.output: Too many open files
Error: open: /tmp/dune0a6e20.output: Too many open files
Error: open: /tmp/dune096c63.output: Too many open files
Error: open: /tmp/dune955bb3.output: Too many open files
Error: open: /tmp/dune2bc4c9.output: Too many open files
Error: open: /tmp/dune5fb45e.output: Too many open files
Error: /tmp/dune887f90.output: Too many open files
Error: /tmp/dune974539.output: Too many open files
Error: /tmp/duneef1640.output: Too many open files
Error: /tmp/dune3efdc3.output: Too many open files
Error: /tmp/dunede8b6d.output: Too many open files
Error: /tmp/dune73cade.output: Too many open files
Error: /tmp/duned68216.output: Too many open files
Done: 3397/3910 (jobs: 1)Fatal error: exception Sys_error("_build/.db: Too many open files")

As @emillon pointed out this seems to be related to #3677: it looks like the null device is repeatedly being opened (and not being closed?). The PR seems to fix the problem for me.

[emillon]

In addition I wonder if we could just close the fd rather than having it open the null device, but that's a more complicated change.

[aalekseyev]

@emillon, closing the fd will lead to unrelated files being assigned the file descriptor 0 and the results may be very confusing. The behavior is much more predictable when the null device is used.

[rgrinberg]

Looks fine. Can you check the fd limit on your machine? Perhaps we should introduce it in CI when running the test suite.

[nojb]

Looks fine. Can you check the fd limit on your machine? Perhaps we should introduce it in CI when running the test suite.

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